Human high molecular weight (HMW) kininogens are now established as a factor required for surface activation of Hageman factor. B4 Beta or B4 gamma kininogen, used at 10 per cent the concentration of HMW kininogen in normal plasma, completely corrects the clotting and other defects of Williams plasma (Kininogen-deficient). The B4 kininogens also enhance the ability of active Hageman factor to activate its three natural substrates, plasma thromboplastin antecedent, and prekallikrein), also the ability of Hageman factor fragments to activate prekallikrein. Pepsitocin, the kinin released from human kininogens by pepsin has been purified to homogeneity. Quantitative amino acid analysis gave. 1.1 (1) Ser, 2.7 (3) Pro, 1.4 (1) Gly, 0.7 (1) Met, 2.2 (2) Phe, 1.0 (1) Lys, 1.9 (2) Arg (theory for Met-Lys-bradykinin in parentheses). Kallikrein has been localized in the distal tubular cells of the kidney cortex by means of fluorescent antibody to rat urinary kallikrein.